A typical application of a voltage regulator is to provide a stable supply voltage to an integrated circuit (IC) in an electronic device, such as a microprocessor in a computer system. Some voltage regulators are used in electronic devices that include ICs that require multiple stable supply voltages. For example, a microprocessor in a computer system can have core circuitry, such as execution, cache, and decode circuitry, that requires a primary stable supply voltage, and peripheral circuitry, such as input/output buffers, that requires a secondary stable supply voltage.
One prior approach to providing multiple stable supply voltages to ICs in electronic devices is illustrated in FIG. 1. In FIG. 1, controller 101, switches 102 and 103, primary inductor 104, and primary capacitor 105 form a typical switching regulator that provides a stable primary output voltage on primary output voltage terminal 106. When the primary output voltage is below a desired potential, controller 101 causes switch 102 to close or remain closed, and causes switch 103 to open or remain open. With switch 102 closed and switch 103 open, current flows through primary inductor 104, from negative terminal 107 to positive terminal 108, to charge primary capacitor 105 and supply current to primary load 109. After the primary output voltage rises to the desired potential, controller 101 causes switch 102 to open and switch 103 to close, such that primary output terminal 106 is connected to ground through primary inductor 104. Then, the voltage induced across primary inductor 104 causes current to continue flowing in the same direction through primary inductor 104, until the primary output voltage falls below the desired potential, which causes controller 101 to close switch 102 and open switch 103.
Also in FIG. 1, transformer 110, including primary inductor 104 and secondary inductor 111, is used along with rectifying diode 111, series regulator 113, and secondary capacitor 114 to provide a stable secondary output voltage on secondary output voltage terminal 115 at a potential above that of the primary output voltage. When switch 102 opens and switch 103 closes, the voltage induced across primary inductor 104 induces a voltage across secondary inductor 111, which forward biases rectifying diode 112 and raises or maintains the input to series regulator 113 above the potential of primary output voltage terminal 106. Since the primary output voltage at which switch 102 opens and switch 103 closes is constant, the voltage induced across primary inductor 104 is a constant, and the voltage induced across secondary inductor 111 is a constant depending on the ratio of the number of turns in secondary inductor 111 to the number of turns in primary inductor 104. However, secondary load 116 on secondary output terminal 115 is variable. When the current load on secondary output terminal 115 is increased, the current and voltage drop across rectifying diode 112 increase. The voltage at the input to series regulator 113 is reduced by the increase in the voltage drop across rectifying diode 112. Thus, one problem with this prior approach is that if the current load on secondary output terminal 115 is increased above a certain point, the voltage at the input to series regulator 113 will be too low, and the secondary output voltage will fall below the desired potential.
A typical solution to this problem with is to increase the number of turns in secondary inductor 111, such that the voltage at the input to series regulator 113 is high enough above the desired secondary output voltage to compensate for a given increase in the current load on secondary output terminal 115. The number of turns in secondary inductor 111 is also typically increased to raise the voltage at the input to series regulator 113 to compensate for the voltage drop across series regulator 113. However, the inefficiency inherent in increasing the number of turns in secondary inductor 111 to raise the voltage at the input to series regulator 113 above the desired secondary output voltage results in increased power consumption.
Therefore, a novel approach to providing multiple output voltages from a voltage regulator is desired.